Polyoxymethylene (POM) is an extremely useful technical plastic, which because of its physical properties finds widespread use in the most various fields.
Polyoxymethylenes in the form of crude polymers generally have inadequate stability and must therefore be subjected to special processing steps before they can be processed into shaped bodies, filaments, films, and the like by conventional shaping methods.
Among the known provisions that may be employed to process and stabilize the crude polyoxymethylene are
a) the deactivation of acidic catalyst components that are needed for the polymerization; PA1 b) the removal of unconverted monomer from the crude polymer; PA1 c) the removal or blocking of unstable chain ends; and/or PA1 d) the incorporation of stabilizers into the crude polymer in order to protect against heat, oxygen, light, acidic substances, and/or other substances harmful with respect to the raw polymeric material. PA1 glycol formaldehyde acetal (1,3-dioxolane), PA1 propane diol formaldehyde acetal (1,3-dioxane), PA1 butane diol formaldehyde acetal (1,3-dioxopan), PA1 diglycol formaldehyde acetal (1,3,6-trioxocane), PA1 hexane diol formaldehyde acetal (1,3-dioxoxane), and PA1 butene diol formaldehyde acetal (1,3-dioxacycloheptene-5). PA1 There is a greater throughput of crude polymers as a result of aggregates that remove unstable chain ends and residual monomers. PA1 The volume of the condensates of the water vapor and of the removed monomers is slight. PA1 There is less burden on monomer recovery systems and hence higher capacities, resulting from higher trioxane contents in the condensates of the demonomerization steps. The condensates can be delivered directly to processing steps for monomers, without the otherwise usual steps of concentrating them.
The aforementioned provisions are employed either alone or in combination. In the known processing and stabilization methods there is variance in how the various provisions are combined as well as the timing of their use in the method.
However, in many methods, particular attention is devoted to the deactivation of acidic catalyst components and the removal of residual monomer. For instance, it is known that the copolymerization of formaldehyde or cyclic oligomers of the formaldehyde, preferably 1,3,5-trioxane, with suitable comonomers such as cyclic ethers and acetals in the presence of cationic polymerization catalysts (U.S. Pat. Nos. 3,027,352 and 3,803,094) is generally incomplete. Depending on the polymerization method and conditions, for instance, from 10 to 50% of the monomers are unconverted in the reaction and remain in the polymerization aggregate, leaving it in gaseous form and/or in a form in which they are bound to the polymer. Isolating and recovering the unconverted monomers entails considerable expenditure of time and effort.
It is known that the processing of polyoxymethylene can be done by deactivating the catalyst used by means of basic additives in an aqueous phase or in an organic solvent, for trioxane, for instance, and by ensuing steps of filtration, washing and drying. This procedure is complicated, and large quantities of solvent are required in order to recover the monomer.
Isolation of the residual monomers from crude polyoxymethylene by treatment with an inert gas at elevated temperature (115.degree. to 170.degree. C.) in the presence of thermal stabilizers and/or gaseous deactivators, such as aliphatic amines, has also already been described. The deactivation, the removal of the residual monomers, and the incorporation of stabilizers are carried out simultaneously in a single method step. The disadvantage of this method is that the residual monomers have to be recovered from large quantities of vehicle gas. Further, the deactivators must be removed in quantity before the recovered monomers are re- used. The treatment times, ranging from 5 minutes to 8 hours, require correspondingly large-sized technical equipment (U.S. Pat. No. 3,210,322).
The use of solvents and deactivators in the gas phase, specifically in a certain temperature range that does not cause depolymerization of the resultant polyoxymethylene copolymers, is also known. In that process, melting of the polyoxymethylene is avoided. For this method, the significance of the temperature is greater, and various temperature ranges are given for different deactivators (German Patent Disclosure DE-OS 33 11 145).
It is also known that crude polyoxymethylenes in the absence of considerable quantities of trioxane, or in other words residual monomers, are broken down by acidic catalyst residues. One method therefore prescribes carrying out the deactivation of the catalyst residues prior to the removal of the residual monomers (U.S. Pat. No. 2,989,509).
As is clear from the above-cited patent applications and patents, the removal of residual monomer, or of other unstable volatile components of the crude polyoxymethylene, and the deactivation can be carried out either separately or in combination. Both steps can have a decisive effect on the quality of the final product.
As a rule, the removal of the unconverted monomer and the deactivation step, which are usually done together, are followed by the removal of unstable chain ends. This term should be understood to mean both the chemical reaction of splitting off of unstable chain ends and their physical isolation, for instance by raising the temperature and degassing.
A combination of residual monomer removal, deactivation, and removal of unstable chain ends is possible in principle. One example is German Patent Disclosure DE 12 46 244.
The German Patent Disclosure DE-OS 14 95 666 relates to a method for stabilizing polyoxymethylenes that contain hemiacetalic terminal groups, in which the raw polyoxymethylene copolymers are treated at temperatures from 100.degree. C. to the sintering point of the polyoxymethylenes at a pressure higher than atmospheric pressure with saturated water vapor that contains from 0.1 to 10% of a volatile, basic catalyst and from 1 to 50% of a swelling agent.
The treatment of the crude polymers according to this reference is carried out essentially in the pressure vessel, while such swelling agents such as alcohol, ketones or ether and basic components such as ammonia or alkylamines are added in order to remove residual monomer and to split off unstable chain ends. In particular, according to DE-OS 14 95 666, no significant terminal group breakdown is achieved if treatment is done at atmospheric pressure.
Other methods describe the melting open of the polyoxymethylenes in the steps of deactivation and/or demonomerization and/or removal of unstable chain ends (DE-OS 37 03 790, DE-OS 37 38 632 and European Patent Disclosure EP 0 137 305 A3).
The targeted removal of unstable components and remaining monomers in a continuous melting process has also been described. In it, the crude polymer together with alkaline-acting compounds is rapidly melted while being intensively kneaded and is transported in the molten state through a zone that is under a vacuum to a degassing apparatus. It is considered that an essential step in this method is the crude polymer predominantly melts open with the aid of mechanical energy (Examined German Patent Disclosure DE-OS 12 46 244), with the addition of the aforementioned compounds that inactivate the catalyst residues. Suitable stabilizers may be added, simultaneously, with one of the processing steps or after it.
From the above-mentioned references, the significance of these processing steps for the commercial utility of the individual method and for the quality of the final products becomes clear. Thus depending on many individual variables (particle size of the crude polymer, temperature, dwell time, type and quantity of deactivator, type and quantity of catalyst, mechanical mixing, presence of stabilizers, etc.), more or less severe damage to the crude polymer can occur, which is expressed for example in a rise in the melting index and in unstable components.
It is also problematic that the residual monomers must sometimes be recovered from relatively large quantities of solvent or vehicle gas. If deactivators are present, then it is always also necessary to perform separation from the residual monomers. Consequently, recovery of the reusable residual monomers is complicated and uneconomical.